Lasers are widely used today in fiber and free space segments for high data rate communication links, remote sensing applications (LIDAR) and more. In these applications the modulated light signal is modulated using electro-optical modulators and demodulated using, usually, electro-optical receiving devices.
In optical communications the modulation scheme commonly used is On-Off Keying (OOK), where only the power of the light is modulated. Alternative modulation schemes include Phase Shift Keying (PSK), where the data is encoded in the phase of the signal. Three types of PSK modulation are currently used in optical communications: Binary Phase Shift keying (BPSK), Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM).
By using such communication schemes, for example, in optical communication systems, the capacity and link performance can be greatly enhanced in comparison with the direct detection schemes. In LIDAR, which is the extension of radar to the optical domain, the required shaping of the pulses can be achieved, such as chirped pulses, Barker coding, etc.
For these applications and others, the light should be modulated both in amplitude and phase, essentially with a complex modulation signal. There is a need in compact, reliable, and low-cost receivers for such advanced modulating schemes.
At the receiver the received optical signal is mixed with the local oscillator signal by an optical interface that is usually based on one or more optical hybrids, such as directional hybrids, polarization splitters, and 90-degrees balanced hybrids. At the output from the optical interface, the optical field is converted into electric currents by one or more PIN photodiodes.
If the local oscillator and the received optical carrier have the same frequency, the electric currents provided by the photodiodes are baseband signals and the receiver is of the homodyne type. Respectively, if the local oscillator and the received optical carrier have different frequencies, the electric currents are shifted to the intermediate frequency (IF).
The present invention relates generally to the integrated phase diversity and polarization diversity optical receiver designated to detect the optical signal, to mix it with another optical signal, to transform the signal into electrical domain for further processing. The present invention also addresses methods of the device fabrication and use.
Optical devices currently available are based on non-integrated and/or semi-integrated solutions, i.e. optical fibers or optical fiber-based components are used for connecting of various electro-optical components and/or splitting/combining the optical signals. An integrated solution for the device that are capable to provide an arbitrary format demodulation (phase and/or amplitude modulation) is disclosed in co-pending U.S. patent application Ser. Nos. 10/669,130 and 11/610,964 commonly owned with the present application, both incorporated herein by references.
The coherent detector in '130 and '964 is an active device integrated on a single chip combined with series of photodetectors. An array of photodetectors need to be precisely positioned relatively the outputs of the optical interface of the receiver. In passive integrated devices, the photodetector alignment may be performed by light transmission through the integrated waveguide and its detection by the photodetector. Maximal intensity of the photodiode current corresponds to the best alignment. This approach is not applicable to active integrated devices. The photodiodes need to be aligned before the device activation. The intensity of the light passing through waveguides is different in the device in active state and the device in non-activated state. The best alignment of the photodiodes for the optical receiver in non-activated state differs from their best positioning in the active device. Proper alignment of the integrated device output waveguides with the photodetectors is crucial for the device performance. There is a need to simplify the alignment during the manufacturing process.